This invention relates to a mixing apparatus for mixing a magnetizable developer material for use in developing images on an image recording surface. The mixing apparatus in accordance with this invention is particularly adapted for use in magnetic brush development systems utilized in reproducing machines.
Magnetic brush development systems have found wide commercial application in xerographic type reproducing machines. Such developer systems utilize a magnetizable developer mix comprised of relatively coarse magnetizable carrier particles and extremely fine colored toner particles. The triboelectric relationship between the carrier particles and the toner particles is such that upon mixing the toner particles adhere electrostatically to the carrier particles. The developer mix is normally applied to the image recording surface by means of a magnetic development roll.
A recognized problem with this type of development system is the tendency for localized toner depletion to occur due to the development of non-uniform images. For example, the development of high density images at a given axial location of the roll causes toner depletion and loss of density for further images. To remedy this problem the prior art has suggested the use of a variety of cross-mixing devices including chutes, baffles and augers. In U.S. Pat. Nos. 3,707,947 to Reichart; 3,697,050 to Stanley; and 3,572,289 to Maksymiak, variously oriented chutes or baffles re-distribute the developer in an axial sense.
A wide variety of magnet arrangements have been utilized for magnetic brush development rolls. U.S. Pat. Nos. 3,543,720 to Drexler and 3,654,902 to Hakanson are illustrative of magnetic brush development roll designs wherein magnetic poles are alternated circumferentially of the roll. This results as described in Hakanson in alternating normal and tangential fields which disrupt and reform the magnetic brush on the roll surface. It is stated that this creates inter-mixing and circulation of the toner mix to allow excess toner in certain portions of the brush to be worked into adjacent areas to equalize the amount of toner along the axial length of the brush. The magnet arrangement of the foregoing patents produces substantially uniform fields axially of the development roll and non-uniform fields circumferentially of the development roll.
In another approach alternate magnetic poles are arranged axially of the magnetic roll to provide non-uniform fields axially of the roll and substantially uniform fields circumferentially of the roll. Illustrative of this approach is U.S. Pat. No. 3,003,462.
Other approaches to providing non-uniform fields axially of the development roll are described, for example, in German Pat. No. 1,218,287 to Kramskins and in U.S. Pat. Nos. 2,846,333 to Wilson; 2,854,947 to Gramo; and 3,196,831 to Sugarman. These patents show the use of a number of different structures for passing magnetic developer along helical or other non-circular paths on a magnetic brush roll.
Magnets have also been used in conjunction with magnetic brush development rolls for controlling the flow of developer material on the roll. For example, in U.S. Pat. No. 3,754,526 to Caudill the use of a pick-up magnet which is shorter in an axial sense than the development magnet causes the developer material to move inwardly of the ends of the roll. Alternatively, two magnet segments inset from the ends of the roll are utilized in place of the shortened single magnet. Likewise, in U.S. Pat. No. 3,648,656 to Ogawa magnetic field generating means are utilized to prevent developer from being picked up on the ends of the development roll. Another example of the use of magnetic field generating devices for controlling the axial width of the developer blanket on the magnetic brush roll is set forth in U.S. Patent application Ser. No. 510,939, filed Oct. 1, 1974, to Hudson. In U.S. Pat. No. 3,915,121, to Wilcox, magnets are used to provide flow baffles for trimming a moving magnetic brush of developer material.